Police say the 150 seriously wounded are among 190 injured in the blasts.

British Prime Minister Tony Blair says the explosions appear to be a coordinated terrorist attack aimed at disrupting the start of the Group of Eight (G-8) summit in Scotland.

"It's reasonably clear that there have been a series of terrorist attacks in London," Blair said. "There are obviously casualties. Both people that have died and people seriously injured, and our thoughts and prayers of course are with the victims and their families."

Blair has left the summit for London.

Home Secretary Charles Clarke says four blasts have been officially confirmed. London's police chief earlier said police believed six blasts had occurred.

A group claiming to represent the Al-Qaeda network claimed responsibility for the attacks in a message posted to an Islamic website.

The message could not be immediately authenticated.

The explosions come a day after London won its bid for the 2012 Olympics.